In assembling a number of optical fibers into an optical fiber cable, a spacer is employed which has helical grooves formed on its outer peripheral surface for receiving the optical fibers. Of conventional spacers of this kind, a spacer of the type having grooves which continuously helically extend in only one direction about the entire circumference of the spacer such that the grooves cover an angular range of 360.degree. is preferable in that even if the spacer is bent after assembling optical fibers into a cable, stresses applied to the fibers lengthwise thereof are substantially equalized. However, this spacer presents a problem as it needs a large-scale device for fitting the optical fibers into the grooves.
That is to say, to fit optical fibers into grooves helically extending around the spacer in one direction, it is necessary that bobbins each having wound thereon an optical fiber be rotated about the spacer, or a cable take-up drum be turned so as to rotate the cable around its axis. In either case, a special device is needed therefor, inevitably resulting in the entire mechanism involved becoming complex, large-scale and expensive.
To overcome the above-noted problem, there have been proposed, for example, a spacer of the type having helical grooves each of which changes its direction to right (clockwise) and left (counterclockwise) alternately at a predetermined angle below 360.degree., and a spacer of the type in which optical fibers set in paper or plastic tapes having V- or U-shaped cross section are helically arranged around a tension member such that each optical fiber covers an angular range of less than 360.degree. while changes its direction to right and left alternately.
With such spacers which have helical grooves changing its direction to right and left alternately, when the optical fiber cable is bent, grooves inside and outside the bent portion differ in length from each other. Accordingly, in the case where optical fibers are disposed on a particular side of the spacer, bending of the cable will lead to scattering stresses that are applied to the optical fibers, introducing difficulties in taking up the cable onto a drum or laying the cable in a bent or curved form, for instance.
That is to say, when the grooves are each formed helically extending around a less than 180.degree. portion of the spacer, optical fibers outside the bend of the spacer will be stretched and the optical fibers inside the bend compressed; this degrades the transmission characteristics of the optical fibers, and hence impairs their reliability.
As a solution to these problem there has also been suggested a spacer in which each groove is formed wide or deep at its direction changing portion to provide therein an extra length for the optical fiber, but this involves additional steps in forming the groove.
It is therefore an object of the present invention to provide a novel optical fiber carrying spacer which permits fitting of optical fibers into grooves by use of relatively simple facilities and remarkably reduces stresses which are applied to the optical fibers in their longitudinal direction when the resulting optical fiber cable is bent.